X-ray detectors for the acquisition of X-rays, in particular quantum-counting X-ray detectors, have a sensor layer which is sensitive to X-rays and which is coupled to an evaluation electronics unit. The sensor layer is conventionally formed from a semiconductor material (for example amorphous selenium, a gallium arsenide or cadmium telluride compound or the like). Upon the incidence of X-rays, electron hole pairs (charge carriers of negatively or positively charged charge) are formed in the sensor layer. The charge carriers in turn each form a positive and negative space-charge cloud.
On account of an electrical voltage applied to the sensor layer the charge carriers are separated from one another and move in each case to the oppositely charged electrodes (arranged on the surface of the sensor layer). As a result of the current caused hereby an (electrical) signal is generated in the evaluation electronics unit which is proportional to the energy of the absorbed X-rays. Notwithstanding this, in the case of a quantum-counting X-ray detector individual X-ray quanta (photons) and where applicable the individual quantum energies thereof are detected.
In order to achieve a sufficiently fast transport of the charge carriers (the space-charge clouds) right through the sensor layer, voltages of approximately −300 V to more than −2,000 V—depending on the thickness of the sensor layer—are applied to the sensor layer (corresponding to an electrical field strength of approximately −10 kV/mm).
Large-area X-ray detectors are required in particular for computed tomography systems. The manufacture of continuous sensor layers having an edge length of several tens of centimeters is however technically very complex and has high associated costs. In order to nevertheless be able to produce large-area X-ray detectors as cost-effectively as possible, a plurality of comparatively small (single) detector elements having the structure described above are arranged side by side. The detector elements typically have a sensor area of between 1 and 4 cm2. In order to achieve as high an image quality (in particular a high resolution) as possible the detector elements are moreover arranged at as small a spacing as possible of for example approx. 0.1-05 mm from one another.
A fault situation can occur during operation of the X-ray detector in which the voltage of a detector element deviates from its normal operating voltage. Such a fault situation is present for example in the event of a defect in the voltage supply associated with the detector element or in the evaluation electronics unit. In this case the operating voltage drops significantly at regular intervals, mostly to a voltage value of 0 V. A fault situation can however also occur in the form of a voltage fluctuation, in particular as a result of an overvoltage, where in the latter case the actual voltage of a detector element significantly exceeds the operating voltage. In both fault situations a voltage difference arises between the faulty detector element and the or each (immediately) adjacent detector element. If the voltage difference exceeds a critical voltage value, a (spark) flashover can occur between the two adjacent detector elements. The sensor layer of both detector elements (as well as the evaluation electronics unit thereof) can be destroyed by the flashover. The danger of spark flashovers in the event of a fault is promoted in this situation in particular by the minimal spatial spacing of the detector elements from one another.